The present invention relates to a method for improving quality in fusion welding operations where protective gas is supplied to the welding point via a hose line.
The invention also relates to an arrangement for use in implementing the method.
In the fusion welding of aluminium, a number of material-specific quality problems arise, which lead to high repair costs. The major and most extensive problem arises as a consequence of the presence of porosity in the weld joint. The main cause of the porosity is that hydrogen, H2, is readily dissolved in liquid aluminium. The dissolved hydrogen accumulates and forms gas bubbles in the solidified metal, which give rise to a strength-reducing porosity in the weld joint. It is therefore desirable to achieve as little porosity as possible in the weld metal.
The most common sources of the hydrogen are moisture, dirt, oil and fat which come into contact with the fused metal during welding. The welding should therefore take place under clean and dry conditions. In spite of the fact that attempts have been made to achieve conditions which are as dry as possible, it has not been possible to overcome porosity in the weld joint caused by moisture.
The present invention is based on the knowledge that a contributory cause of such porosity resulting from moisture may be condensate which for various reasons is formed on the inside of protective-gas hoses and welding guns. This moisture is then conveyed forward by the protective gas to the pool at the welding point. Tests performed have also confirmed that this is the case.
In other welding operations, such as welding stainless steel, titanium, high-alloy stainless steel, nickel-based steel and magnesium, problems associated with inter alia oxidation which is difficult to explain arise. With the knowledge that has been gained in the development of the present invention, it has emerged that these problems also may be due to impurities which originate from the inside of the protective-gas hose and are supplied to the pool by the gas flow. Such impurities may consist of oxygen and also moisture which diffuse in through the walls of the hoses used for the protective-gas flow. Other impurities present in the hoses can also influence the weld quality.
The main object of the present invention is to produce a technique which considerably reduces the quality impairments which occur in fusion welding using protective gas as a consequence of impurities brought along by the protective gas from the inside of the protective-gas hoses.
According to the present invention, this is achieved by virtue of the fact that a small flush-gas flow is made to flow through the protective-gas hoses even when the normal protective-gas flow is shut off. In this way, moisture is prevented from condensing on the inside of the hose, and other impurities are prevented from accumulating there when the protective-gas flow is shut off.
The particularly characteristic features of a method and an arrangement according to the present invention emerge from independent claims. Further features of the invention are indicated in the associated subclaims.